Swingarm off, 1/2 an inch thick road grime and an o-ring in there from somewhere:
That looks quite a bit like mine did!! I used a putty knife to scrape out the bigger chunks of stuff. Then I got an old tooth brush and a can of mineral spirits (paint thinner) and went to town scrubbing as well as I could.
Since you have the swing arm off, I would encourage you to inspect it very closely for cracks. I've read on this very forum about cracks around the shock mount. I've also seen pictures of a cracked pivot pinch. In my own experience with buying an old Supersport, I found my aluminum swing arm to be cracked right at the pivot point.
Where the solid cast piece meets the rectangular tube of the swing arm. Right on the welds. Invisible with the bike sitting there, or even with the swing arm off. It took removal of the chain slider and a trip through a steam cleaner to find the cracks in mine.
It was cracked across both the top and bottom welds as well as down inside weld that is hidden by the engine. I thought I'd stumbled on something but found out from a fellow forum member that his had cracked in the exact same manner and place as mine did.
It isn't something that is easy to find, specially when the entire area is covered in old flung off chain lube like the counter shaft area in your pic. Apparently though, it is more common than I'd originally thought.
Do a search for posts by T8KC and take a look at the engine stand he uses. It's simple and effective. I made one inspired by his and it's been a huge help. Alternatively, another member welded up some square tube and fabbed an adapter to mount to a standard automotive engine stand.
My engine was also filthy due to the leaking front forks. Suspension fluid coated most of it, and of course dirt and dust stick to that so it was a real mess. I've cleaned it, cleaned it again and am still cleaning crap out of it.
I had to make myself a small "shovel" out an old coping saw blade because there was so much garbage stuck in the fins of the horizontal cylinder. I literally shoveled dirt and gravel out of the top fins on it for an hour or more. Still haven't gotten all of it out either. Even found a wayward roll pin stuck in between the fins. I dunno if came from the bike or was just kicked up by the front tire.
The swing arm has since been welded. Removing the swing arm bearings was a huge PITA. I've detailed how to do that in a thread in the "Mechanical Techl" section of the forum. It's in the Hall of Wisdom sub forum should you find yourself facing that dilemma.
I've looked at mine several times and thought it would be soooo much easier to just soak the thing down in simple green and blast away with my pressure washer. And it would be....if I didn't care about getting water into everything. So, it's back to using the miniature shovel I made, and a toothbrush with some mineral spirits to get things clean enough to maybe one day use some simple green to clean it up and spray some fresh paint on the parts that need it.
Keep up the great work getting that thing clean and serviced.....sean